Steam reforming is the most widely used method for generating synthesis gas from light hydrocarbons. In steam reforming, a hydrocarbon feed such as natural gas, liquid gas or naphtha is endothermically reacted with steam in a catalytic tubular reactor, hereinafter called “steam reforming reactor”, to form synthesis gas. Process heat and flue gas heat are utilized for steam generation. Details of such a system may be found, for example, in Chapter 2 of the article “Gas Production” in Ullmann's Encyclopedia of Industrial Chemistry (doi:10.1002/14356007.a12_169.pub2, electronic edition 2007).
First, superheated process steam is added to the purified, in particular, desulphurized, hydrocarbon feed in accordance with the steam/carbon ratio required for the reforming reactions. Then the hydrocarbon/steam mixture obtained is heated and distributed over the catalyst-filled and externally-fired tubes of the steam reforming reactor. When the mixture flows through the tubes, it reacts in accordance with the following reactions:CnHm+nH2O→nCO+((n+m)/2)H2  (1)CH4+H2OCO+3H2  (2)CO+H2OCO2+H2  (3)
In order to minimize the methane content in the synthesis gas, maximize the hydrogen yield and avoid the formation of elemental carbon and deposition thereof on the catalyst, the steam reforming reactor is operated at a higher steam/carbon ratio than is theoretically required.
For purposes of the following description, the materials and mixtures described, for example a hydrocarbon feed or a synthesis gas, are referred to a “streams” or “fractions”. A stream normally describes a fluid flowing through a conduit, whereas a fraction designates a portion of a starting mixture separated from the starting mixture.
A stream or a fraction can be described as “rich” or “lean” in one or more components, e.g. hydrogen, wherein “rich” is generally a portion more than 75% and “lean” a portion less than 25% in each case based on a weight or volume basis.
The steam reforming process is generally followed by a plurality of workup steps which serve for obtaining pure end products from the synthesis gas. The synthesis gas is first cooled followed by a step of removing carbon dioxide in a carbon dioxide separation unit, wherein some of the carbon dioxide is scrubbed out, for example with methanol and/or diethanolamine. Carbon dioxide can also be removed, for example, by subsequent temperature-swing adsorption. A cryogenic separation of the synthesis gas is performed to provide a hydrogen-rich fraction and a carbon monoxide-rich fraction, with the latter being compressed and delivered to the battery limit.
The hydrogen-rich fraction resulting from the cryogenic separation, still contains impurities such as carbon monoxide, carbon dioxide and light hydrocarbons, such as methane, ethane, propane, ethylene and propylene. These impurities are separated in a pressure-swing adsorption plant, obtaining residual gas or tail gas, and the hydrogen product, preferably pure hydrogen from the hydrogen-rich fraction.
Since the heat balance for the main reactions (1)-(3) cited above is endothermic, the heat required must be supplied by external firing. Fuel gas for this firing can be supplied from the residual gas of the pressure-swing adsorption plant, and from also heating gas from beyond the battery limit.
The effect of various parameters on the composition of the synthesis gas are summarized in WO 2005/040704 A2 and known methods are disclosed in WO 03/086965 A1, EP 1 544 166 A2 and EP 0 790 212 A1.
Depending on the desired hydrogen/carbon monoxide quantitative product ratio in the synthesis gas, a carbon dioxide fraction from the carbon dioxide separation unit can be recycled via a recycle compressor and added to the hydrogen feed. In a plant having a natural gas feed, with complete recycling of the carbon dioxide fraction that is separated, and depending on further boundary conditions, a hydrogen/carbon monoxide quantitative product ratio of approximately 2.5 can be obtained. If less hydrogen is required, carbon dioxide can be imported from beyond the battery limit, in order to decrease the hydrogen/carbon monoxide quantitative product ratio.
If a higher hydrogen/carbon monoxide quantitative product ratio is desired, the recycling of the carbon dioxide fraction can be reduced to zero. This results in the hydrogen/carbon monoxide quantitative product ratio increasing to approximately 4.1. If more hydrogen is to be produced, the plant can be operated using different parameters (e.g. higher steam fraction) but this has a direct effect on plant size and therefore capital costs. As an alternative, a side stream shift or a separate shift line can be installed, but this also leads to an increase in the capital costs.
There remains a need in the art for improvements to the production of hydrogen-rich synthesis gas using a stream reforming reactor without an appreciable increase in capital costs.